The invention relates to constant current source circuits, and more particularly to constant current source circuits that permit large output voltage swings at the terminal through which the constant current is supplied, and still more particularly to amplifier circuits that use the constant current source circuits as load devices.
FIG. 1A shows a prior art circuit of the type referred to as a regulated cascode current mirror. It produces an output bias current I.sub.OUT flowing through terminal 2. N-channel MOSFET M2 is a "cascode" transistor having its gate coupled to conductor 3, which is the junction between a constant current source I2 and the drain of N-channel MOSFET M4, which is an optional cascode transistor having its gate connected to a bias voltage V.sub.BIAS. MOSFETs M3 and M4 and constant current source I2 constitute an inverting amplifier producing an output voltage on conductor 3 in response to an input voltage on conductor 4. Optional cascode MOSFET M4 increases the gain of that inverting amplifier, and therefore increases the output impedance at conductor 2. MOSFETs M1 and M5 and constant current source I1 constitute a conventional current mirror. The current I1 is "mirrored" through the drain electrode of MOSFET M1 to produce the constant current I.sub.OUT.
The circuit shown in FIG. 1A is also illustrated in FIG. 1B, wherein the inverting amplifier including MOSFET M3 and current source I2 is designated as an amplifier 5 having a gain equal to -A.
A shortcoming of the regulated cascode current mirror circuit 1 shown in FIGS. 1A and 1B is that the voltage on output conductor 2 cannot swing over as wide a range as is desirable. Specifically, the lowest voltage that can appear on conductor 2 is equal to the sum of the gate-to-source voltage of MOSFET M3 and the drain-to-source voltage of MOSFET M2. Any lower voltage on conductor 2 turns MOSFET M3 off, making the circuit inoperative.
So-called CMOS gain enhancement operational amplifiers suffer from low output voltage range or swing, i.e., from low dynamic range. To solve this problem, various complex CMOS gain enhancement operational amplifiers have been used, including those having single-ended, current-mirroring, or fully differential structures. Because of their complexity, such complex gain enhancement amplifiers have had low gain-bandwidth products and high power dissipation, due mainly to parasitic capacitances associated therewith.
Thus, there is an unmet need for a CMOS gain enhancement operational amplifier which has higher output dynamic range, wider bandwidth, lower power dissipation, and requires less chip area than has been achievable in the prior art.